1. The Technical Field
The present invention is directed to electrical circuit substrates, particularly electrical circuit substrates including transparent electrodes and other electrodes and circuit elements. The present invention is further directed to fabrication of such substrates and fabrication of electrical circuits using such substrates.
2. The Related Art
Transparent touch panel substrates and transparent circuitry, for example, indium tin oxide (ITO) electrodes or traces on a glass panel or flexible substrate, are known in the art of touch panel design. These features can improve a user interface by allowing the user to view, for example, decoration or other indicia, through the touch panel and allowing panel backlighting to reach the user.
Though such panels often are desirable, their designers are faced with certain obstacles. For example, transparent conductors generally exhibit poor solderability characteristics and, therefore, are not well-suited for receiving and connecting to other electrical circuit components, such as resistors, capacitors, transistors and integrated circuits. Also, transparent conductors are not ideal electrical conductors. Indeed, the conductivity of ITO, a commonly used transparent conductor, is generally inferior to that of copper or other commonly used electrical conductors. For this reason, designers often limit use of transparent conductors to those areas of an touch panel where transparency is required, and they generally prefer to use conventional conductors, such as copper, where transparency is not required.
However, difficulties arise in implementing the numerous interfaces that may exist between transparent and conventional circuit portions. For example, transparent and conventional circuit portions often are built on separate substrates which are subsequently connected physically and electrically. Connecting such separate substrates together requires precise alignment which can be adversely affected by stack up of tolerances among the various components to be joined. Connecting separate substrates together also requires precise joining techniques, such as use of compression connectors, anisotropic adhesives, and silver or other metal filled ink to bridge transparent and other circuit portions. Once joined, separate boards connected in this manner are prone to electrical and/or physical separation after initial assembly and during use due to handling, vibration, and differential shrinkage and expansion between the two boards. Further, application of the foregoing techniques tends to limit the minimum pitch or spacing between individual touch pads, thus placing limits on the compactness of an overall touch panel.
Attempts have been made to incorporate both transparent and conventional conductive circuit portions on a single substrate. However, these attempts have involved application of a transparent conductive layer over a conventional conductive layer using screen printing process and/or serial patterning and etching of thin films. For example, one such attempt involves applying a thin film of copper to a substrate, plating additional copper onto the copper film, patterning and etching the copper layer, applying a thin film of transparent conductive material to the substrate and conventional circuit portions and then patterning and etching the transparent conductive material layer. These steps involve various different processes that traditionally are carried out on different production lines. As such, this technique is relatively time consuming and costly. Further, the resulting structure inherently yields sharp transitions at junctions between transparent and conventional circuit portions because of the nature in which the transparent layer overlaps the conventional circuit portions. These sharp transitions result in unreliable electrical connections between the transparent and conventional circuit portions.